Willy L. Bohn

Review: Laser-Ablation Propulsion

LASER ablation propulsion (LAP) is a major new electric propulsion concept with a 35-year history. In LAP, an intense laser beam [pulsed or continuous wave (CW)] strikes a condensedmatter surface (solid or liquid) and produces a jet of vapor or plasma. Just as in a chemical rocket, thrust is produced by the resulting reaction force on the surface. Spacecraft and other objects can be propelled in this way. In some circumstances, there are advantages for this technique compared with other chemical and electric propulsion schemes. It is difficult to make a performance metric for LAP, because only a few of its applications are beyond the research phase and because it can be applied in widely different circumstances that would require entirely different metrics. These applications range from milliwatt-average-power satellite attitude-correction thrusters through kilowatt-average-power systems for reentering near-Earth space debris and megawatt-to-gigawatt systems for direct launch to lowEarth orbit (LEO). We assume an electric laser rather than a gas-dynamic or chemical laser driving the ablation, to emphasize the performance as an electric thruster. How is it possible for moderate laser electrical efficiency to givevery high electrical efficiency? Because laser energy can be used to drive an exothermic reaction in the target material controlled by the laser input, and electrical efficiency only measures the ratio of exhaust power to electrical power. This distinction may seem artificial, but electrical efficiency is a key parameter for space applications, in which electrical power is at a premium. The laser system involved in LAP may be remote from the propelled object (on another spacecraft or planet-based), for example, in laser-induced space-debris reentry or payload launch to low planetary orbit. In other applications (e.g., the laser–plasma microthruster that we will describe), a lightweight laser is part of the propulsion engine onboard the spacecraft.

Lightcraft experiments in Germany

Vertical flight and pendulum experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode. Pulsed laser energy of up to 240 J/pulse was delivered from a highly reproducible e-beam sustained CO2-laser at repetition rates up to 45 Hz. The lightcraft mass was varied in the range between 22 and 55 g. An average thrust of 1.1 N has been derived from the flight data and the highest impulse coupling coefficient found in the pendulum experiments was 33.3(DOT)10-5 Ns/J. A double shock wave was detected that leaves the thruster exit and an attempt was made to model the thrust, using a modification of Sedovs similarity solution for a blast wave. Finally, the propulsion requirements for the launch of a 10 kg mass into low Earth orbit are presented.

Laser Propulsion Activities in Germany

Activities related to laser supported propulsion concentrate on investigations of the fundamental phenomena arising from the interaction of high‐power CO2 laser pulses with a simple bell engine. Breakdown dynamics, plasma, and lightcraft accelerations are carefully measured using optical and laser diagnostics. In a first‐order approach the expanding plasma can be described by the point explosion model with counterpressure. Wire guided vertical flight experiments in the laboratory have been undertaken and analyzed. Comparative impulse measurements in ambient air and ablated material are presented for a series of experiments performed either at atmospheric pressure or at reduced pressures down to vacuum. Impulse coupling coefficients, average exhaust velocities,specific impulse, and jet efficiencies are derived from the experimental data. The repetitively‐pulsed CO2 laser device used in all experiments shows a potential of achieving 50 kW average power. Finally, long‐term perspectives of laser propulsion wi...

Laser lightcraft performance

Using a pulsed CO2 laser test-bed the properties of a laser lightcraft are investigated with respect to the thrust generating mechanisms. New optical diagnostics allow to measure the mechanical impulse imparted to the lightcraft by the laser pulse with high accuracy. Density variations are measured with a HeNe probe laser and identified as shock waves originating from the laser-induced air breakdown. Wire guided indoor vertical flights are demonstrated with accelerations of about 1 g. Satisfactory agreement between theory and experiment in the time-space domain is found using a strong explosion model. Prediction of mission requirements suggests that lightcrafts of 1 - 10 kg can be placed into orbit with current technology.

Two-Micron Thulium-Pumped-Holmium Laser Source for DIRCM Applications

There is an increasing need for the generation of mid-infrared radiation in the 3 to 5-micron region especially in the absorption minima of the atmospheric windows. Recent progress in heat seeking detector technology, operating in these atmospheric windows, make it necessary to develop compact and reliable mid-infrared laser systems that can be installed in airborne platforms. Future DIRCM systems will be equipped with high repetition rate/low energy per pulse lasers as well as low repetition rate/high energy per pulse lasers. We report on the development of a Tm:YLF-fiber laser (1.908 &mgr;m) pumped Ho:YAG (2.09 &mgr;m) high energy laser system with pulse energies up to 90 mJ at pulse lengths close to 20 ns and operating at 100 Hz. Using single mode fiber lasers as end-pumped sources for the master-oscillatorpower- amplifier (MOPA) system almost diffraction limited beam quality resulted. The frequency conversion into the 3 to 5-micron region is performed with a zinc germanium phosphide (ZGP) crystal in a linear or ring resonator. Propagation of the mid-infrared laser beam through moderate turbulent atmosphere will be simulated numerically using phase screens and Fresnel transformation.

Supersonic COIL operation at DLR, Germany

The German Aerospace Research Establishment (DLR) is routinely operating its supersonic chemical oxygen-iodine laser (COIL). Meanwhile over 200 single tests have been performed at run times extending to 1 minute. Power levels of 5 KW have been exceeded. Parametric studies were performed resulting in chemical generator efficiencies of about 43% for the baseline operation with high power output. The BHP molarity was found as one of the most important parameters for a stable and reproducible operation. Yield measurements revealed lower numbers than expected from theoretical calculations. The paper gives an overview of the COIL device and discusses the experimental and calculated results of the investigations.

High-power CO-overtone laser

A modified electron beam controlled pulsed CO2 laser is used as a multi spectral multi purpose test bed in order to generate high power fundamental and first overtone laser transitions in CO. The revisited concept includes an all solid state power supply which provides a highly reproducible operation at pulse repetition frequencies of up to 100 Hz. The active gas mixture is recirculated in a closed loop and kept at near room temperature using conventional water cooling. Discrimination of the CO fundamental band is obtained by using specially coated dielectric mirrors and introducing additional intracavity diaphragms. Unprecedented laser pulse energies of 25 J are reported in the overtone transitions covering a spectral range between 2 micrometers and 3.5 micrometers . Further scaling of pulse energies is expected in the near future using larger diameter resonator mirrors.

Investigations on the efficiency of a rotating disk type oxygen generator.

For a rotating disk type generator the dependencies of utilization and yield on the generator operating mode are experimentally investigated. The fundamental effects of rotational speed, reduced gas volume, gas flow and liquid phase composition on the efficiency of singlet delta oxygen generation are discussed together with theoretically predicted generator performace from literature.

Multikilowatt supersonic chemical oxygen-iodine laser

The Chemical Oxygen Iodine Laser (COIL) is an attractive candidate for easy power scaling at short wavelengths. High specific power output from supersonic operation leads to compact devices. The German Aerospace Research Establishment (DLR) has recently completed construction and assembly of a multikilowatt supersonic COIL and started experimental investigations at its Lampoldshausen rocket test site. The excited oxygen is produced by a rotating disk generator, provided by the Phillips Laboratory, Albuquerque, NM. Currently the laser is operated without a cold trap. After the injection of the iodine, the laser gas is expanded to an isotropical Mach number of 1.8 by a multi-element grid nozzle. At present, laser power is extracted with a single pass stable resonator located immediately after the nozzle exit plane. Pressure is recovered by a supersonic diffusor and a 3-stable pumping system.

Laser interaction with materials: introduction.

Laser-materials interaction is the fascinating nexus where laser physics, optical physics, and materials science intersect. Applications include microdeposition via laser-induced forward transfer of thin films, clean materials processing with femtosecond beams, creating color filters with nanoparticles, generating very high density storage sites on subpicosecond time scales, structuring solar cell surfaces for higher efficiency, making nanostructures that would be impossible by other means, and creating in-volume waveguiding structures using femtosecond laser filaments.